1. Field of the Invention
The present invention relates to the field of reducing quantizing distortion of previously quantized electronic signals in systems which do not pass the quantized signal with full quantizing accuracy. The invention will be found to be useful for previously quantized analog signals, either in digital form, such as digital pulse amplitude modulation which is commonly known, or in analog form. Analog forms include multilevel pulse amplitude modulation, where the analog signal is constrained to a given number of predetermined values. Such multilevel pulse amplitude modulation is used for example to transmit analog signals in the vertical and horizontal blanking interval areas of a video signal, wherein each given analog level corresponds to a particular binary pattern representation of the original analog signal. While the invention is described by way of example for use with video signals, one skilled in the art will recognize that it may also be used with other types of signals, such as audio or telemetry signals. In the description of the preferred embodiment provided herein, reference is made to timing signals such as horizontal sync, etc., which are somewhat restricted to video type signals, one skilled in the art will recognize that the descriptions herein referencing these intervals may be adapted to fit other timing signals, such as blocks of audio data, radar sweep periods, telemetry scan periods, etc.
The invention finds particular usefulness in applications where a video signal which has been previously digitized with 10 bit accuracy is passed through equipment having only 8 bit capability. The quantizing distortion introduced by the truncation of the lower two bits of resolution is reduced by offsetting the 8 bit truncated version by a small amount, preferably 1 LSB in a regular and predictable pattern, with the pattern being controlled in accordance with the value of the truncated bits. The invention which is particularly useful for digitized video signals and associated equipment will also find use for other types of quantized signals and with other levels of quantizing precision and truncation, so long as a clocking signal, related to the quantizing of the continuous analog version, or representing a continuous analog version, is available.
2. Description of the Prior Art
The prior art contains circuits for masking the quantizing distortion of video signals. Many of these circuits add random noise to the reduced precision signal, or use a variety of rounding schemes to achieve the reduction in distortion. One of these rounding schemes is described in the book D.multidot.I.multidot.G.multidot.I.multidot.T.multidot.L Television Edited by C. P. Sandbank, published by John Wiley & Sons, New York, N.Y. 10158. Pages 553 through 555 describe a method whereby a 12 bit digitized video signal is added to itself in an error feedback circuit to provide an 8 bit video signal. This method works well for the linear ramp signal shown in the book, however it is not nearly as effective on real video signals.